Diurnal Variation in 

Trace-Metal Concentrations in Streams 

Trace metals in streams can be 

toxic to fish and aquatic insects. 

Therefore, sampling streams for 

metals is an important aspect of 

water-quality monitoring. In the 

past, scientists assumed that a 

stream water sample collected at 

any time of the day would provide 

an accurate assessment of metal 

concentrations on that day assuming 

streamflow was relatively constant. 

Recent studies, however, have 

shown that dissolved concentrations 

of some trace metals exhibit 

substantial and consistent variation 

throughout the day (fig. 1). The 

magnitude and widespread 

occurrence of these diurnal (24-

hour) concentration patterns, or 

cycles, was unexpected. This 

discovery of the consistent 

occurrence of diurnal metal cycles 

has significant implications for how 

we study and monitor the 

environment. 

DIURNAL METAL CYCLES 

Daily variations in dissolved

metal concentrations are shown in 

figure 1 for the South Fork Coeur 

d’Alene River in Idaho. These 

variations are examples of diurnal 

metal cycles. Two important 

characteristics of diurnal metal 

cycles are the time of day when the 

minimum and maximum

concentrations occur and the 

magnitude of the change in metal 

concentration. In the South Fork,

the timing of the diurnal cycles for 

cadmium, manganese, and zinc was 

similar. Concentrations of these 

metals increased during the night, 

reaching the highest values shortly

after sunrise. Concentrations then 

decreased during the late morning 

and early afternoon, reaching the 

lowest values during mid to late 

afternoon.

Assessing concentrations of trace metals in streams is found to be significantly more complex than previously 

thought. New research indicates that concentrations of metals dissolved in stream water vary by time of day. 

Scientists and 

government agencies will 

need to consider diurnal 

metal cycles to ensure 

accurate assessment of 

metal concentrations in 

streams. 

_______________________________________________________________________________________________________________________________________________________________________________

U.S. Department of the Interior Fact Sheet FS-086-03 

U.S. Geological Survey October 2003

South Fork Coeur d’Alene River, Idaho (site 2 on fig. 4) 
Diurnal metal cycles for 

Prickly Pear and High Ore Creeks 

in Montana are shown in figure 2. 

Note that the timing of diurnal 

cadmium, manganese, and zinc 

cycles in these streams was similar 

to the cycles for these metals in the 

South Fork Coeur d’Alene River 

(fig. 1). However, the timing of 

diurnal arsenic cycles was the 

opposite, with maximum concentrations in the late afternoon and 

minimum concentrations in the 

early morning. 

The potential magnitude of 

diurnal metal cycles is shown by the 

data for Prickly Pear and High Ore 

Creeks (fig. 2). Diurnal cycles for 

zinc were the largest, with 

concentrations changing as much as

500 percent in Prickly Pear Creek. 

Cadmium and manganese cycles 

were intermediate, with changes as 

much as 120 and 290 percent, 

respectively, in High Ore Creek. 

Diurnal variations in arsenic 

concentrations (as much as 54 

percent in Prickly Pear Creek) were 

proportionally much less than the 

variations for cadmium, manganese, 

and zinc. 

Diurnal metal cycles occur over 

a wide range of concentration 

levels. For instance, diurnal zinc 

cycles were found at concentrations 

greater than 1,000 μg/L in the South 

Fork Coeur d’Alene River (fig. 1) 

and at concentrations less than 80 

μg/L in Prickly Pear Creek (fig. 2). 

Units of micrograms per liter (μg/L) 

are equivalent to parts per billion. 

Trace-metal concentrations in 

streams exhibit diurnal cycles 

routinely and regularly. For 

example, during diurnal sampling 

episodes in 1995 and 1997 on High

Ore Creek, dissolved zinc concentrations were relatively high (fig. 3). 

Cleanup efforts conducted upstream

at an historical mine site after the 

1997 sampling reduced the amount 

of zinc entering the stream, and zinc 

concentrations were lower during

samplings in 1999-2001. These 

data show that the diurnal zinc cycle 

persisted and that the timing 

remained the same, even though the 

general concentration level changed 

substantially during the 6-year 

period. 

Data on diurnal metal cycles for 

13 streams in Montana and northern 

Idaho (fig. 4) are presented by

Nimick and others (2003). The data 

document and confirm the 

widespread occurrence of diurnal 

metal cycles. The streams had 

gravel beds and were typical of 

mountain headwater streams in the 

northern Rocky Mountains. The 

streams varied in size, with the 

smallest having streamflow of about 

0.5 cubic feet per second (ft3

/s) and 

the largest having streamflow of 

270 ft3

/s at the time of sampling. 

One aspect common to diurnal 

metal cycles is that they occur in 

streams that have neutral to slightly

alkaline pH, which is typical of 

most streams in the Nation. These 

types of diurnal metal cycles have 

not been observed in acidic streams 

more directly affected by mine 

drainage.
Diurnal metal cycles have 

previously been reported in a few 

instances. Diurnal cycles in 

dissolved arsenic concentrations 

were measured in Whitewood 

Creek, South Dakota (Fuller and 

Davis, 1989), and in the Madison 

and Missouri Rivers, Montana 

(Nimick and others, 1998). Bourg 

and Bertin (1996) and Brick and 

Moore (1996) were the first to 

document diurnal zinc cycles. 

IMPLICATIONS 

Water-quality data commonly

are collected to characterize metal 

concentrations in streams. These 

data are used to establish baseline 

environmental conditions, indicate 

the locations of important metal 

sources, plan and evaluate cleanup 

of contaminated sites, detect longterm trends in metal concentrations, 

and evaluate potential risks to fish 

and other aquatic organisms. 

However, if diurnal variability of 

metal concentrations is substantial 

and persistent, such evaluations 

likely are, at least, much less certain 

than previously thought and, at 

worst, potentially misleading or 

wrong. In the future, the 

implications of diurnal metal cycles 

will be an important consideration 

when designing plans for waterquality sampling and evaluating 

historical data on metal concentrations. 

Collecting water-quality samples in Prickly 

Pear Creek, Montana (near site 7 on fig. 4) 
What Causes Diurnal Metal Cycles? 

A number of physical, chemical, and biological mechanisms potentially can explain diurnal cycles in dissolved 

metal concentrations. These mechanisms include: 

• Diurnal cycles of sorption of metals to the surfaces 

of streambed material 

• Diurnal cycles of formation and dissolution of 

minerals containing metals 

• Diurnal cycles of uptake of metals by growing 

aquatic plants 

• Diurnal variation of the input of metals from an 

upstream source 

• Diurnal changes of geochemical conditions within 

the streambed 

• Diurnal variation of streamflow 

This Fact Sheet was prepared by David A. Nimick and is based on the journal article: 

Nimick, D.A., Gammons, C.H., Cleasby, T.E., Madison, J.P., Skaar, Don, and Brick, C.M., 2003, Diel cycles in 

dissolved metal concentrations in streams—Occurrence and possible causes: Water Resources Research, 

v. 39, no. 9, 1247, doi:10.1029/2002WR001571. 

Suggestions for Additional Information: 

Bourg, A.C.M., and Bertin, Clotilde, 1996, Diurnal 

variations in the water chemistry of a river 

contaminated by heavy metals—Natural biological 

cycling and anthropic influence: Water, Air, and Soil 

Pollution, v. 86, p. 101-116. 

Brick, C.M., and Moore, J.N., 1996, Diel variation of 

trace metals in the upper Clark Fork River, Montana: 

Environmental Science and Technology, v. 30, 

p. 1953-1960. 

Fuller, C.C., and Davis, J.A., 1989, Influence of coupling 

of sorption and photosynthetic processes on trace 

element cycles in natural waters: Nature, v. 340, 

p. 52-54. 

Nimick, D.A., Moore, J.N., Dalby, C.E., and Savka, 

M.W., 1998, The fate of geothermal arsenic in the 

Madison and Missouri Rivers, Montana and 

Wyoming: Water Resources Research, v. 34, 

p. 3051-3067.

__________________________________________________________________________________________________________________________________

For more information, contact: 

District Chief 

U.S. Geological Survey

3162 Bozeman Avenue 

Helena, MT 59601 

1-406-457-5900

or 

1-888-ASK-USGS 

Please visit the USGS on the Internet: 

The USGS Montana District homepage is: 

 http://mt.water.usgs.gov/ 

The USGS Toxic Substances Hydrology Program 

 homepage is: http://toxics.usgs.gov/ 

The National USGS homepage is: 

 http://www.usgs.gov/ 

Sorption best explains diurnal metal cycles for two reasons. First, it explains the concurrent timing of the high 

and low dissolved metal concentrations found daily in streams (fig. 1). Second, it is the only mechanism that 

explains the opposite timing of the arsenic concentration cycles relative to the concentration cycles of the other 

metals (fig. 2). 

Sorption is a chemical reaction in which metals are transferred between stream water and the surfaces of 

streambed materials, such as rocks and aquatic plants. During desorption, metals are detached from streambed 

materials and added to stream water, thereby increasing dissolved metal concentrations in stream water. During 

adsorption, metals are transferred from stream water to streambed materials, thereby decreasing dissolved metal 

concentrations. 

Sorption is affected by the temperature and pH of stream water. Water temperature and pH commonly 

increase in streams during the day and decrease during the night in response to the daily cycles of daylight and 

darkness. These changes in temperature and pH are key factors in determining the amount of each metal that is 

adsorbed or desorbed. 

Sorption of a specific metal ion is affected by its charge. Arsenic ions are negatively charged whereas 

cadmium, manganese, and zinc ions are positively charged. Therefore, arsenic desorbs when the other metals 

adsorb. This opposite behavior explains the opposite timing of the diurnal arsenic cycles relative to cadmium, 

manganese, and zinc cycles (fig. 2).